1. Field of the Invention
The invention relates to a rotary-anode X-ray tube, including a sleeve bearing having bearing faces which are rotatable relative to one another in a direction of rotation and at least one of which is provided with a pattern of grooves.
2. Description of the Related Art
An X-ray tube of this kind is known, for example from EP-A 378 274 which corresponds to U.S. Pat. No. 5,077,775, or from DE-OS 28 52 908. The sleeve bearings of these tubes can operate in a given direction of rotation only. Between the bearing faces which are rotatable relative to one another there is provided a liquid lubricant, for example a gallium alloy.
During steady operation of the sleeve beating in the relevant direction of rotation, such pressure conditions arise in the lubricant, inter alia under the influence of the groove pattern, that a hydrodynamic lubricant film is formed and the bearing surface cannot contact one another. At the beginning and at the end of the rotation, however, these conditions are not satisfied so that wear phenomena which limit the service life of the bearing may appear.
It is an object of the present invention to construct an X-ray tube of the kind set forth so that the service life of the sleeve bearing is prolonged. This object is achieved in accordance with the invention in that a recess is provided at the rearmost end of the grooves, viewed in the direction of rotation.
The invention is based on the recognition of the fact that particles become detached from the bearing surfaces during the start and landing processes; these particles will also be referred to hereinafter as abrasion particles. The abrasion particles collect at the areas of the grooves in which the highest pressure occurs in the lubricant during rotation of the bearing. This is the rearmost pan of the bearing, viewed in the direction of rotation. In the course of time such a large amount of abrasion particles could collect and be compressed in this part of the grooves that extensive wear can occur at that area, resulting in a breakdown of the bearing.
When the grooves are provided with a recess at that area, a substantially larger quantity of abrasion particles can collect at that area, so that the service life of the bearing is prolonged. The recess, extending over only a comparatively small pan of the overall length of the grooves, has an insignificant effect only on the bearing capacity of the bearing and can be taken into account right from the start of design. The recess should be provided only in the grooves, but not in the ridges between the grooves.
It is to be noted that from EP-A 204 359 there is already known a sleeve bearing comprising a groove pattern with a varying depth of the grooves. This bearing is a sleeve bearing for a horizontally positioned shaft which is journalled in a bearing sleeve. The groove depth therein varies continuously in the circumferential direction so that at the area where the bearing is exposed to the highest load, the depth of the grooves is smallest. As a result, the bearing has its highest bearing capacity at that area. However, the aim is not to prolong the service life of such a bearing nor is such a prolongation achieved by the described steps.
In an embodiment of the invention, one of the bearing faces is provided with a groove pattern in two adjoining areas, the grooves in one area extending towards the grooves in the other area at an angle which deviates from 180.degree. C. and which is preferably an acute angle, said grooves terminating at the boundary between the areas and a recess being provided in the grooves at their end near the boundary. In another embodiment of the invention, however, one of the bearing faces is provided with a pattern of grooves which extend in the direction perpendicular to the axis of rotation, which grooves start at the outer side and terminate at the inner side, viewed in the direction of rotation, the inner ends of the grooves opening into a ting-shaped or circular recess. In this case the rear part of the grooves, viewed in the direction of rotation, is situated at its inner end, so that the recess must be provided at that area.